diff --git a/README.md b/README.md
index 3d3b6b4..d154135 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@ This project depends on internal APIs used by the GitHub Actions Cache.
 See `gha-cache/README.md` for more details on how to obtain the required tokens.
 
 ```
-cargo run -- -c creds.json
+cargo run -- -c creds.json --upstream https://cache.nixos.org
 cargo build --release --target x86_64-unknown-linux-musl
 cargo build --release --target aarch64-unknown-linux-musl
 nix copy --to 'http://127.0.0.1:3000' $(which bash)
diff --git a/nix-actions-cache/src/main.rs b/nix-actions-cache/src/main.rs
index 6360ef8..b9c481b 100644
--- a/nix-actions-cache/src/main.rs
+++ b/nix-actions-cache/src/main.rs
@@ -58,12 +58,20 @@ struct Args {
     /// Using another version string allows you to "bust" the cache.
     #[arg(long)]
     cache_version: Option<String>,
+
+    /// The upstream cache.
+    ///
+    /// Requests for unknown NARs are redirected to this cache
+    /// instead.
+    #[arg(long)]
+    upstream: Option<String>,
 }
 
 /// The global server state.
 #[derive(Debug)]
 struct StateInner {
     api: Api,
+    upstream: Option<String>,
 }
 
 #[tokio::main]
@@ -91,7 +99,10 @@ async fn main() {
         api.mutate_version(cache_version.as_bytes());
     }
 
-    let state = Arc::new(StateInner { api });
+    let state = Arc::new(StateInner {
+        api,
+        upstream: args.upstream,
+    });
 
     let app = Router::new()
         .route("/", get(root))
@@ -160,7 +171,11 @@ async fn get_narinfo(
         return Ok(Redirect::temporary(&url));
     }
 
-    Err(Error::NotFound)
+    if let Some(upstream) = &state.upstream {
+        Ok(Redirect::temporary(&format!("{}/{}", upstream, path)))
+    } else {
+        Err(Error::NotFound)
+    }
 }
 async fn put_narinfo(
     Extension(state): Extension<State>,
@@ -193,7 +208,11 @@ async fn get_nar(Extension(state): Extension<State>, Path(path): Path<String>) -
         return Ok(Redirect::temporary(&url));
     }
 
-    Err(Error::NotFound)
+    if let Some(upstream) = &state.upstream {
+        Ok(Redirect::temporary(&format!("{}/nar/{}", upstream, path)))
+    } else {
+        Err(Error::NotFound)
+    }
 }
 async fn put_nar(
     Extension(state): Extension<State>,